Development version of kvmclock FreeBSD driver for virtual machines.

References and prior art:

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/virtualization_deployment_and_administration_guide/chap-kvm_guest_timing_management
https://www.vmware.com/files/pdf/techpaper/Timekeeping-In-VirtualMachines.pdf
http://oliveryang.net/2015/09/pitfalls-of-TSC-usage/#33-tsc-emulation-on-different-hypervisors
https://reviews.freebsd.org/D29531
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=216759

WWW: https://reviews.freebsd.org/D29733